A device such as a printer and projection television, which displays a color image requires a light source with the three colors R (red), G (green) and B (blue).
Recently, as a light source of this type, a wavelength conversion laser (laser oscillator) has been developed which uses laser beams of the 900 nm band, 1 μm band and 1.3 μm band as the fundamental wave laser beams, and converts the fundamental wave laser beams to the second harmonics with half the wavelengths (double frequency) using a nonlinear material (SHG: Second Harmonic Generation).
To extract the laser beams with the desired wavelengths at high efficiency, it is necessary for the SHG to achieve high conversion efficiency from the fundamental wave laser beams to the second harmonic laser beams.
To convert the fundamental wave laser beams to the second harmonic laser beams within the wavelength conversion element, phase matching conditions must be satisfied between the fundamental wave laser beams before the conversion and the second harmonic laser beams after the conversion.
The phase matching conditions are conditions that correct the phase shift between the fundamental wave laser beams and the second harmonic laser beams between the wavelength conversion element.
As an element that carries out the wavelength conversion while satisfying the phase matching conditions, a quasi phase matching (QPM) wavelength conversion element using a periodic structure is known, for example.
The QPM wavelength conversion element has an optical waveguide formed in a periodically poled lithium niobate (PPLN) which is a nonlinear optical crystal, and inverts its polarization periodically along the waveguide direction.
The following Patent Document 1 proposes, as for a planar waveguide-type wavelength conversion element that has a flat nonlinear optical material and propagates the fundamental waves of laser beams in a plurality of laser oscillation modes in the direction perpendicular to a flat main surface perpendicular to the optical axis to perform the wavelength conversion of the fundamental waves, a wavelength conversion element and a wavelength conversion laser wherein the nonlinear optical material has non-polarization inversion regions and polarization inversion regions formed therein by varying their polarization inversion periods in such a manner that the nonlinear optical material has a phase matching bandwidth including the phase matching conditions of at least two laser oscillation modes among the plurality of laser oscillation modes.
The wavelength conversion element in the Patent Document 1 modifies the polarization inversion periods of the nonlinear optical material to obtain the phase matching bandwidth including the phase matching conditions of at least two laser oscillation modes. Thus it can carry out the wavelength conversion of the fundamental wavelengths for at least two laser oscillation modes.